Blow-by gases from engine combustion tend to seep past an engine's piston rings and work their way into the engine's crankcase. Positive crankcase ventilation (PCV) involves recycling these blow-by gases through a valve, called a PCV valve, to the intake manifold where they are pumped back into the cylinders for another opportunity at combustion. When an engine is idling the air pressure in the intake manifold is lower than the air pressure in the crankcase, and it is this lower pressure that draws the blow-by gases through the PCV valve and into the intake manifold. As the engine load increases, the air pressure in the intake manifold increases and there is less suction, reducing the amount of blow-by gases recycled to the cylinders.
In highly efficient and highly loaded internal combustion engines designed to operate with reduced pumping losses, the intake manifold pressure can be very close to atmospheric pressure, resulting in substantially reduced intake vacuum that is too low for efficient operation of a conventional PCV system.
Accordingly, it is desirable to provide a positive crankcase ventilation system for efficiently purging blow-by gases from the crankcase of an internal combustion engine designed to operate with substantially reduced intake vacuum.